Combustion process and gas burner with low nox, co emissions

ABSTRACT

The process obtains a plurality of small flames which become violet when combusting natural gas. In particular the amount of primary air induced is at least 80% of the air stoichiometrically required for combustion; the secondary air laps all sides of each singly small flame arising from the small group of slots on the burner body, in the area where each flame leaves the surface of the burner body so as to swell it making its width at least as big as its height and/or alternatively the contact surface between each flame and the secondary air is increased right from the first steps in combustion, so that each small group creates a small flame with two divergent bladed wings, similar to &#34;butterfly wings.&#34;

FIELD OF THE INVENTION

This invention relates to a process and equipment to obtain efficientcombustion of gas by means of an atmospheric burner.

BACKGROUND OF THE PRIOR ART

Burners for gaseous fuels are generally divided into fan-assistedburners and atmospheric burners.

In general, in using atmospheric burners an imperfect combustion mayoccur, with harmful emissions of carbon monoxide and nitric oxides whichcan cause atmospheric pollution.

EP-A-0009831 refers to an atmospheric burner (POLIDORO) in which theperforated surface of a cylindrical burner diffuser consists oftransverse groups comprising subgroups of slots spaced out 2 mm in atransverse direction, with pilot holes at each end. Each sub-groupconsists of a densely-packed series of parallel transverse slot withcenter distance of only 1.2 mm, in order to obtain a "single flamefront" from each group, i.e., a big vertical flame, with the aim ofreducing burner noise and providing high thermal power.

Also known, per EP-A-0217470, is an atmospheric burner (NEFIT) in whichthe primary air-gas mixture, drawn in through a Venturi tube, is thendirected downwards at the outlet of this tube. The flow is then fedback, up at the side with turbulence reduced to a minimum, to feed theslots in a perforated diffuser. The purpose is to obtain combustionwhich is as uniform as possible, "without any disturbing noise beingproduced". In fact (see figures) the surfaces of the diffuser of thebody present groups of slots similar to EP-A-0009831.

Also, DE-A-2132968 (FOGLIANI, PANINI and VECCHI), teaches covering of anatmospheric burner equipped with small groups of slots arranged closetogether and/or rows of holes, aligned transversely, with the relativepilot holes at the ends. The burner body has a polygonal cross-section,in particular featuring a diffuser with two angled sides which, as shownin FIG. 2, have slots across the joint between these sides, creating adouble-horned flame on a transverse plane of the diffuser itself. Theflame in this burner is more stable and noise level is reduced.

However, prior art does not envisage the reduction of harmful emissionsof nitrogen oxides and carbon monoxide. This may be justified by thefact it was considered impossible to solve the problem of reducingpollutant emissions from atmospheric burners simply by acting on thecombustion method or on the proportioning of the burner itself.

SUMMARY OF THE DISCLOSURE

The object of this invention is to provide a process and an atmosphericburner to obtain gas combustion having very low levels of harmfulemissions.

This and related objects are realized by providing a combustion process,in a novel atmospheric burner.

The novel process involves the steps of:

aspiring, by means of a Venturi tube, an amount of primary air equal toat least 80% of the air stoichiometrically required for combustion;

enabling a flow of secondary air to contact the gas-primary air mixtureimmediately after ignition to bring the total air involved to a levelabove the stoichiometric value; and

starting and completing combustion within the thickness of a blade of awing-shaped flame providing a luminous emission of violet color withinthe visible spectrum, at a wavelength below 0.42 micron, from thecombustion of natural gases.

BRIEF DESCRIPTION OF THE DRAWING

The present invention is described on the basis of some exemplaryembodiments illustrated in the attached drawings, in which:

FIG. 1 shows a longitudinal sectional view of a burner;

FIG. 2 shows a cross-sectional view along the line A--A in FIG. 1;

FIG. 3a and 3b show of possible variations in the slot groups;

FIG. 4 shows a view of a possible arrangement of the slot groups;

FIG. 5 shows a variant of FIG. 4;

FIG. 6 shows a plan view of the upper part of a burner;

FIG. 7 shows a plan view of a variant of the burner in FIG. 6;

FIGS. 8 and 9 show variants of FIG. 2;

FIGS. 10a, 10b, 10c and 10d show views of possible variants of thesub-groups of holes in which the mutual positions of a sub-group ofslots and a sub-group of holes is shown.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The bladed flame is a new phenomenon. In reality, the bladed flame isnot vertical, nor big or compact, but divergent small and thin.

This new process reduces harmful emissions to extremely low levels, evenin combustion chambers whose thermal power, in reference to their basearea, is at least 40 W/cm². Even if it could be known, in general terms,that it was possible to supply the burner with a quantity of primary airexceeding 80% of the stoichiometric air value, given the availability ofsecondary air to bring the total above the stoichiometric parameter,however the burner bodies produced according to the prior art were notof a type which would function satisfactorily in these conditionsbecause of the overheating of the thin sheet metal of which the burner'sdiffuser is constructed. Moreover, the prior art did not allow theconstruction of burners with low harmful gas emissions for combustionchambers with thermal power exceeding 40 W/cm² of base area. Inaddition, if a thermal power rating of 1.1 kW/cm² of burner's diffusermixture outlet area was exceeded, it would then become impossible todraw in a quantity of primary air exceeding 80% of the stoichiometricvalue. The values of thermal power (equal to specific combustioncapacity) were generally 1.3-1.7 kW/cm², which allowed the risk ofoverheating to be eliminated.

The mixture in the prior art devices travelled at high speed inside theburner, leading to high load losses and making it impossible to draw inlarge quantities of primary air.

The gas burner implementing the present invention with the burnerdiffuser made of thin sheet metal, e.g. of thickness around 0.5 mm, isequipped with a Venturi tube having a parabolic intake, with all partsproportioned aerodynamically to allow infeed with primary air in aproportion of over 80% of the stoichiometric requirement. Itsspecifications are: thermal power not exceeding 1.1 kW/cm² of area ofthe ports for the outflow of the mixture from the burner diffuser,air-gas mixture average speed not exceeding 4.5 m/s. Preferably, theair-gas mixture average speed is 4 m/s in the neck of the Venturi tubeand, between the tube and the diffuser, has an average speed notexceeding 2.5 m/s and is preferably 2 m/s.

The slots in the burner diffuser are arranged in aligned transversegroups, each group being appropriately subdivided longitudinally intotwo subgroups of slots lying not very close together. Each group therebygenerates a double bladed flame which may be characterized as beinggenerally of a butterfly-wing shape, having its center lineperpendicular to the longitudinal axis of the burner and its lowervertex on the stretch of burner between the two sub-groups of slots.

This specially shaped flame allows secondary air to come into contactwith it on all sides over a wide surface area, providing completecombustion in the bladed flame from the very first moment. By contrast,in conventional flames there are no less than three zones in whichcombustion gradually occurs: an internal low-temperature area(blue-green in colour) where the air-fuel mixture is heated to theignition temperature; an intermediate zone (blue in colour) whereincomplete combustion of the mixture occurs, leaving residues of carbonmonoxide and hydrogen; and a single external zone which is violet incolour, where the gases not burnt in the intermediate zone undergocombustion as they come into contact with the secondary air.

The groups of slots are arranged in rows perpendicular to the burneraxis. The distance between each group of slots in any one row isequivalent to at least 65% of the length of the longest slot. The axialdistance between two rows of slots is at least nd/2, where d is theaxial length of the group and n is the number of groups in a row, or thenumber of groups in two adjacent rows when the groups are staggered toform a chess-board type lay-out.

The number of slots in any one group must be between 2 and 5. Moreover,the slots may be arranged so that the outline of the flame halocorresponds to the exposed profile of the heat-exchanger. In addition toproviding excellent heat-exchange conditions, this reduces CO emissionsto a minimum. The stretch between the to sub-groups of slots in eachgroup must be between 2.4 and 2.8 mm, and this ensure the generation ofstable bladed flames.

When burning natural gases, the application of this method in a burneraccording to this invention generates bladed flames of violet colour:measurements made have given wavelength values below 0.42 micron, whichis within the violet field of the visible spectrum.

In the case of atmospheric burners having thin sheet-metal diffusers ofthe type according to this invention, the bladed flame is the only shapeproviding minimal emissions of NO_(x) and CO. This, therefore,represents the achievement of the object of this invention.

The bladed flame, when it is violet in colour for combustion of naturalgases, is the phenomenon which indicates the effect of a new, highlyadvantageous combustion method in the burner according to the invention.

The prior art does not provide atmospheric burners with bladed flames;quite the contrary, in order to increase thermal power and flamestability, the known burners incorporate features intended to group theflames together into solid vertical formations as far as possible, toprovide the so-called "single front". POLIDORO and FOGLIANI, PANINI andVECCHI fall into this category; NEFIT, while not discussing thequestion, however shows a similar slot lay-out. This, in fact, reducesthe flame-secondary air contact surface considerably.

On the other hand, the burner which implements the method described inthe invention is designed to increase the flame-secondary air contactsurface to the greatest possible extent. It generates thin individualflames, each with a large surface area beginning from the stretchbetween the two sub-groups, with a couple of divergent wings whichremain completely separate.

The great advantage in relation to the prior art is the achievement ofhigh thermal power without harmful emissions, and without overheating ofthe burner.

In fact, the burners according to the prior art, provided with groups orsub-groups of slots, with the body designed in such a manner to allow anintake of primary air certainly below 80%, did not generate bladedflames, producing harmful emission.

If the air intake is not coaxial to the Venturi tube, the latterpreferably should be slightly bent with respect to the injector axis inorder to obtain concentricity with the diverted mixture flow.

In some cases, in order to facilitate the backflow of the mixture comingout from the Venturi tube, louvers on the latter are kept open to obtaina gradual backflow. In particular, these louvers are open on the Venturitube in zones far from the flame ports of the burner diffuser.Preferably, the upper part of the burner diffuser, where the slots insmall groups are located, has a bending radius that is larger than theradius at a lower part.

Preferably, the number of slots in each group of a row decreases towardsthe burner's vertical symmetry plane. Moreover the slots can be parallelto each other. In particular, small groups are created by slots ofdifferent lengths, outward decreasing. For example, the area occupied bya small group can be a rhombus. This is favorable in general, andparticularly so with a chess-board configuration.

So that the halo of the bladed flame may follow the shape of the heatexchanger, the rows made up of small groups are longitudinally locatedat a variable distance. It is also possible to vary the distance amongthe groups of a single row. Moreover each group can have a differentport area.

The flame lift preventing holes are numerous and at a certain mutualseparate according to need. In particular, and they may be placed at theapexes of a square and/or equilateral triangle at a distance of 1.3 to1.5 mm, preferably 1.4 mm.

FIGS. 1 and 2 show a burner consisting of a burner body 1 containinginside it a Venturi tube 2, which has a smooth, funnel-shaped mouthpiece3 with a parabolic form. The Venturi tube 2 leaves a free area (hatchedin FIGS. 2, 8 and 9) inside the burner body 1, such as to give anaverage air/gas mixture speed not exceeding 2 m/sec. The upper part ofthe burner body has holes out of which comes a series of small flames F.The section of the burner body 1 where the holes are located is convex,with a bending radius greater than the bending radius of the lower part.See FIG. 2. The holes are formed as small groups 5 of slots 7, eachconsisting of two sub-groups 4 forming a flame F with two wings (FIG.7). The small groups 5 are arranged in rows 8 perpendicular to theburner axis 10. Flame lift preventing holes 6 are provided between thevarious small groups 5 (FIGS. 10a to 10d). The rows of small groups canbe arranged alongside one another with the small groups lined up asshown in FIG. 4, the distance "a" between the small groups 5 in thiscase being at least 0.65b, where "b" is the maximum length of the slotsin a group, while the distance "c" between the rows is at least nd/2,where "n" is the number of small groups making up the row and "d" is thewidth of the totality of slots 7 forming a small group 5. Alternativelythe rows 8 of small groups 5 can be offset with respect of rows 8,, asshown in FIG. 5. with this chess-board configuration, the distances "a"and "c" must comply with the conditions described earlier; "n" in thiscase is the sum of the small groups in the two adjacent, offset rows 8and 8'. The distance between the sub-groups 4 of each small group 5 is2.4-2.8 mm.

Each sub-group may consist of slots 7, equal in length and parallel(FIGS. 3a, 4 and 6); the number of slots can be two, three or four.Alternatively, the sub-group (FIG. 3b) may consist of slots 7' ofdifferent lengths (decreasing from the center of the group outward) sothat the area occupied by each group corresponds to a rhombus. In thiscase the number of slots making up the sub-group may be two, three, fouror a maximum of five.

The flame lift preventing holes 6 can be arranged as illustrated inFIGS. 10a to 10d, i.e., at the apexes of a square and/or an idealequilateral triangle, their center distance I being from 1.3 to 1.5 mm.,preferably 1.4 mm.

Lastly, in the burner shown in FIG. 7 the rows of small groups 5 areseparated at varying distances D1, D2, D3, D4, D5, D6.

This makes it possible to adapt the flame halo 11 to follow the shape ofthe heat exchanger. A similar adaptation is achieved transversely to theburner by varying the number of slots in each small group (FIG. 9) orspacing out the small groups of a single row.

Louvers 12 are provided far from small groups 5 in order to distributethe backflow along the burner body 1. The particular shape and colour ofthe flame makes it possible to achieve combustion of a high capacity ofgas--40 W per square cm of combustion chamber measured in a planview--with a very low level of harmful emissions.

In this disclosure, there are shown and described only the preferredembodiments of the invention, but, as aforementioned, it is to beunderstood that the invention is capable of use in various othercombinations and environments and is capable of changes or modificationswithin the scope of the inventive concept as expressed herein.

I claim:
 1. A process to obtain the combustion of a gas while generatingproducts of combustion having a very low content of harmful emissions(NO_(x), CO), by combustion in an atmospheric burner having a diffuserportion, the process comprising the steps of:providing a quantity ofcombustible gas and sucking therewith into the atmospheric burner aquantity of ambient air to form an air-gas mixture which comprises atleast 80% of the air stoichiometrically required for the combustion;burning the air-fuel mixture while providing secondary air, which fromthe beginning of the combustion laps all sides of each of a plurality ofsingle bladed small flanges (F) exiting from each of a plurality ofgroups of slots on said diffuser portion of the burner, in such a waythat the total amount of air thus involved in the combustion exceeds astoichiometrical air value; and completing combustion of the gas withinthe thickness of the flames corresponding to the groups of slots so asto cause the emission of violet light therefrom of a wavelength lessthan 0.42 micron.
 2. The process according to claim 1, wherein:eachgroup of slots produces a small flame (F) with two divergent bladedwings.
 3. The process according to claim 1, wherein:a distance betweenthe small flames (F) is selected so as to ensure that the halo height ontop of the flames follows the shape of the heat exchanger.
 4. A burner,for burning a gas so as to generate products of combustion that are lowin harmful constituents, comprising:a suction tube comprising a Venturitube portion; and a slotted and pierced diffuser having a plurality ofsmall groups of slots and flame lift preventing holes located in rowsperpendicular to a longitudinal axis of the burner, a distance betweensaid small groups in each row being at least 0.65b, where "b" is thelength of the longest slot of the slots in each small group a distancebetween adjacent rows being at least nd/2 where "d" is the width in anaxial direction of the slots forming each small group and "n" isselected to be one of the number of small groups in a row and the numberof small groups in two adjacent rows in an arrangement of rows where therows are offset, the specific combustion capacity of the burner notexceeding 1.1 KW per each cm² of port area on the diffuser.
 5. A burneraccording to claim 4, wherein:the Venturi tube has a parabolicmouthpiece shaped so that the sucked air maintains a uniformacceleration throughout the mouthpiece, the maximum average mixturespeed being less than 4.5 m/sec., while the burner is sized to be wideenough to maintain an average speed of the flowing mixture under 2.5m/s.
 6. A burner according to claim 4, wherein:the rows of small groupsare offset, thus forming a chess-board configuration.
 7. A burneraccording to claim 4, wherein:the small groups of slots are split in twosub-groups, each formed to have not less than two nor more than fiveslots formed very close to each other, the distance between saidsubgroups being in the range 2.4-2.8 mm.
 8. A burner according to claim4, wherein:the small groups of slots each comprise parallel slots havingdifferent lengths, the longest slot being at the inside of each groupand the shortest slots being at the outside.
 9. A burner according toclaim 4, wherein:the distance between adjacent rows varieslongitudinally along the burner.
 10. A burner according to claim 4,wherein:the distances between said small groups varies across thediffuser.
 11. A burner according to claim 4, wherein:the total portareas of the small groups decrease toward a vertical plane of symmetryof the burner.
 12. A burner, according to claim 4, wherein:the Venturitube is slightly bent with respect to the jet axis in order to provideconcentricity to the flowing mixture of air/gas.
 13. A burner accordingto claim 4, wherein:louvers are provided on the Venturi tube to providea gradual backflow to the air/gas flow.
 14. A burner according to claim4, wherein:an upper part of the diffuser, where the slots in smallgroups are located, has a bending radius larger than radius of the lowerpart of the diffuser.
 15. A burner according to claim 4, wherein:holesare provided at the apexes of one of a square and an equilateraltriangle at a center distance (I) between 1.3 and 1.5 mm.